Detailed structural knowledge of transparent and semi-transparent objects can be gained by measuring the reflective backscattering of light from various layers in the objects. Such structural knowledge can be useful in various industrial applications such as semiconductor chip fabrication. Furthermore, knowing the precise structure of the eye of a human patient is useful in diagnosing certain conditions of the eye, including, for example, glaucoma.
German Patent No. 3201801A1 to Fercher discloses a method referred to as partial coherence interferometry ("PCI") in which light having a short coherence length is combined with a Michelson interferometer to locate the positions of reflecting surfaces within an object. In PCI, a measurement light beam from the interferometer is directed against a particular reflecting surface in the object, and a reference light beam is directed against a known reference surface. The position of the reflecting surface in the object is determined by matching the length of the known reference path that is traversed by the reference light beam to the unknown object path length that is traversed by the measurement light beam.
The above discussed PCI method results in the generation of so-called optical A-scans, which can be thought of as plots of backscattered light intensity as a function of depth within the object. U.S. Pat. No. 5,321,501, incorporated herein by reference, discloses a technique referred to as optical coherence tomography ("OCT") in which several A-scans are combined to effectively map the depth of an object.
The invention disclosed in the '501 patent is embodied in the commercial OCT instrument sold by Humphrey Instruments/Carl Zeiss. Unfortunately, the Humphrey-Zeiss instrument permits measuring only longitudinal sections, not transverse sections, and furthermore it requires that the section geometry be defined a priori. The present invention understands that it is desirable to measure both longitudinal and transverse sections to thereby generate a three dimensional map of the object, and that it is also desirable that the section geometry not be defined a priori. Moreover, the present invention recognizes that because the reference surface of the Humphrey-Zeiss instrument is not part of the object to be measured, the precision of the instrument can be degraded by axial movement of the object during measurement.
In addition to the above-mentioned drawbacks, the present invention recognizes that the speed of measurement of prior instruments is relatively slow, and that prolonged measurement time has undesirable consequences, as set forth in the following discussion. As mentioned above, the object path length is matched with the reference path length in PCI applications, including OCT. This matching, when it occurs, is indicated by the presence of interference fringes caused by the interference of the return reference beam with the return measurement beam. In early PCI applications, the interference fringes were visually detected, which significantly lengthened the time required to gather the backscattering data at the various layer depths. Unfortunately, prolonged data gathering periods limits the resolution of the data when the object being analyzed moves. In the case of the human eye, microsaccidic eye movements tend to limit the resolution of the data.
Accordingly, to facilitate more rapid detection of interference fringes, Hitzenberger et al., in an article entitled "Eye Length Measurement by Laser Doppler Interferometry ("LDI")", Int'l Conf. on Optics within Life Sciences, Garmisch-Partenkirchen, 1990, propose detecting the fringes by heterodyning. Specifically, the Hitzenberger et al. article discloses moving a reference mirror at constant speed to cause a Doppler frequency shift in one of the beams, causing the generation of a detectable "beat" frequency when the reference beam and measurement beam interfere with each other as they return from the object being measured.
As recognized by the present invention, however, while Doppler-based heterodyne detection is an improvement over the visual detection method, the use of mechanically moving parts nevertheless limits the speed of measurement by limiting the magnitude of the induced frequency shift, which is proportional to the speed of the reference mirror. Additionally, the present invention recognizes that mechanically-based heterodyning techniques can induce a varying beat frequency, causing demodulation complications, and also requiring a relatively large filter bandwidth (used during demodulation) to account for the variations. The large filter bandwidth in turn reduces the signal to noise ratio of the instrument.
Accordingly, it is an object of the present invention to provide a method and apparatus for generating a map of a transparent or semi-transparent object. Another object of the present invention is to provide a method and apparatus for rapidly generating a map of a human eye. Still another object of the present invention is to provide a method and apparatus for rapidly generating a map of a human eye that is easy to use and cost-effective, and that is not degraded by axial motion of the eye during measurement.